1. Field of the Invention
The present invention relates to an illumination optical system and a projector provided with the same.
2. Description of the Related Art
FIG. 11 is a perspective view showing the appearance of a typical projector. Here, a rectangular parallelepiped projector 501 includes an upper case 503 which defines the upper surface thereof and is provided with operating buttons 502 disposed thereon, a lower case 504 which defines the lower surface of the projector 501 and a front case 505 which defines the front surface of the projector 501. There is a projection lens 506, the front end portion of which protrudes from the front case 505.
A known optical system in such a projector is constituted, for example, as shown in FIG. 12.
That is, the projector is provided with an illumination optical system 520 for making an illumination distribution of light from a light source 510 uniform and making the light incident on liquid crystal panels 550R, 550G, 550B with the light having the same polarization. The projector also includes a color beam splitting optical system 530 for splitting a beam W emitted from the illumination optical system 520 into red, green, and blue beams R, G, B, respectively, and a relay optical system 540 for guiding the blue beam B out of the beams split by the color beam splitting optical system 530 to the liquid crystal panel 550B associated with the blue beam B. The projector further includes the three liquid crystal panels 550R, 550G. 550B as light modulating means for modulating the respective color beams according to given image information, a cross dichroic prism 560 as a color beam synthesizing optical system for synthesizing the modulated respective color beams and a projection lens 506 for expanding and projecting the synthesized beams on a projection screen.
As shown in a schematic view of FIG. 13 showing an action of the illumination optical system, in the illumination optical system 520, light emitted from the light source 510 is split by a first lens array 521 into a plurality of sub-beams and made incident on a polarization conversion element array 523 via a second lens array 522. Then, the polarization conversion element array 523 is allowed to provide the respective sub-beams with the same polarization. Thereafter, the sub-beams are superimposed on the image forming area of the liquid crystal panels 550R, 550G, 550B by a superimposing lens 524.
By this action, the illumination optical system 520 uniformly illuminates the respective liquid crystal panels 550R, 550G, 550B by one type of polarized light and brightens all over the projection screen when an image is displayed by a projector or the like, thereby contributing to provision of a sharp high contrast all over the image.
However, when an image is projected in a size smaller than preset size by utilizing such a device having higher brightness, unnecessarily excessive light is projected on a projection screen so that the image becomes too glaring to see. It is considered that a projection lens is provided with a variable diaphragm as means for solving this problem. As a result, however, design freedom of the projection lens is significantly limited, that is, the size of the projection lens is increased and the types thereof are limited and so on.